@Inna22 since you opened the door, you could offer as much as you would have spent on Champagne, maybe round up-- or, in your shoes I would probably ask them what they think an appropriate discount would be and hope they have some shame.
FWIW, like @Clay29 I would not use poison. We've had an ongoing mouse problem in one of our cottages-- this summer we installed ultrasonic repellers and since then we haven't had any, knock wood.
@Emiel1 my husband and I once stayed in a gorgeous villa in Hawaii that was in complete disrepair. When it rained water came through the ceiling onto the master bed and there were mice. One night when we came home and I sat on the toilet to pee a mouse ran across my feet. We did not ask for any discount.